PATCHY PROTON AURORA AT MARS: FIRST IMAGES, MULTIPLE
MORPHOLOGIES, AND THREE NEWLY OBSERVED FORMATION
MECHANISMS
Mike Chaffin, LASP, University of Colorado, Boulder, Colorado, U.S.A. (michael.chaffin@colorado.edu),
Chris Fowler, West Virginia University, Physics and Astronomy, Morgantown, West Virginia, United States,
Justin Deighan, Sonal Jain, Greg Holsclaw, LASP, University of Colorado, Boulder, Colorado, U.S.A., Andréa Hughes, Goddard Space Flight Center, Greenbelt, MD, USA, Robin Ramstad, Yaxue Dong, Dave
Brain, LASP, University of Colorado, Boulder, Colorado, U.S.A., Hoor AlMazmi, UAE Space Agency, Abu
Dhabi, United Arab Emirates, Scott England, Virginia Polytechnic Institute and State University, Aerospace
and Ocean Engineering, Blacksburg, VA, United States, Matt Fillingim, Rob Lillis, Space Sciences Laboratory, University of California Berkeley, Berkeley, CA, United States, Fatma Lootah, Mohammed Bin Rashid
Space Center, Al Khawaneej, United Arab Emirates, Krishnaprasad Chirakkil, Susarla Raghuram, LASP
and Khalifa University of Science Technology and Research, Space and Planetary Science Center, Abu Dhabi,
United Arab Emirates, Jim McFadden, Space Sciences Laboratory, University of California Berkeley, Berkeley, CA, United States, Jasper Halekas, Department of Physics and Astronomy, University of Iowa, Iowa City,
Iowa, USA, Jared Espley, Goddard Space Flight Center, Greenbelt, MD, USA, Nick Schneider, LASP, University of Colorado, Boulder, Colorado, U.S.A., Hessa AlMatroushi, Mohammed Bin Rashid Space Center, Al
Khawaneej, United Arab Emirates
We present the first definitive observations of
patchy proton aurora at Mars, indicating spatially
variable precipitation of the solar wind across the
full dayside of the planet under some rare (likely
radial) upstream solar wind conditions. We also present more confined patchy aurora seen at other times
when the upstream solar wind conditions are typical,
which require formation mechanisms that exploit
plasma turbulence related to specific interactions of
the planetary induced and crustal field magnetosphere with the shocked solar wind. These results
establish new forms of proton aurora variability,
encompassing those previously proposed and extending our knowledge of possible formation conditions.
Some of the aurora we observe open the possibility
of tracking some upstream solar wind properties via
auroral emission alone, and others suggest that at
times the solar wind directly interacts with the entire
planetary ionosphere, with unknown impacts on atmospheric heating and loss.
Introduction
Proton aurora at Mars were discovered due to
anomalously bright Lyman alpha 121.6 nm emission
in far-ultraviolet limb scan observations of the thermosphere by the MAVEN Imaging Ultraviolet Spectrograph (IUVS) [Deighan+2018]. These aurora have
a CO2 scale height, consistent with their generation
by incident H Energetic Neutral Atoms (ENAs),
which charge exchange upstream of the bow shock
in the extended H corona of the planet, bypass the
magnetic barrier that deflects the charged solar wind,
and collide with the bulk thermosphere, producing
detectable penetrating protons [Halekas+2015] and
H emissions as a byproduct of collisional excitation
and ionization of the incident ENA. A subsequent
study by Hughes+2019 showed that proton aurora
occurs nearly constantly on the dayside during
Southern Summer, but that it is much less frequent in

other seasons, with dayside occurrence rates of
~15%. Crismani+2019 speculated that some intermittent proton aurora visible during some but not all
of a selected MAVEN periapsis occurred as a result
of radial interplanetary magnetic field (IMF) conditions, which can destabilize the ordinary quasisteady-state interaction of the induced magnetosphere with the solar wind. However, these observations were ambiguous between variations in time and
space and could not be attributed to a unique location
along the instrument line of sight, making causal
interpretation of the auroral variation difficult.
Observations
The Emirates Ultraviolet Spectrograph (EMUS)
[Holsclaw+2021] on the Emirates Mars Mission
(EMM) [Amiri+2021] routinely observes the Martian disk in the far and extreme ultraviolet. As part of
its science mission, the instrument makes images of
the planet in H Lyman alpha at 121.6 nm and H
Lyman beta at 102.6 nm, both of which are sensitive
to H ENA / proton auroral emission. The high sensitivity of the spectrometer, required to enable detection of the distant oxygen corona, makes the instrument an extremely effective detector of auroral activity on the planet.
On multiple occasions over the course of Mars
Northern Summer (Ls ~90-180), EMUS has observed patchy structure with spatial scales of less
than 300 km in dayside emissions at both 102.6 and
121.6 nm, with no corresponding variation in longer
wavelength emissions at 130.4 nm and 135.6 nm,
which are sensitive to neutral O and ionospheric variability on the planet. The occurrence of small-scale
emission variation in wavelengths emitted by the H
atom, on spatial scales much smaller than could be
produced by neutral atmosphere variability in H density or temperature, makes production of this patchy
emission by incident protons or H ENAs a near cer-

tainty. We observe fine-scale spatial variation during
a single 6 second integration along the EMUS slit,
unambiguously establishing that at least some intermittent proton aurora previously observed by IUVS
are likely due to spatial variation. The traditional
mechanism for proton aurora formation due to spatially uniform H+ charge exchange upstream of the
bow shock under normal solar wind conditions is
incapable of producing this small-scale variability,
requiring novel formation mechanisms.
Interpretation
We observe multiple qualitatively different morphologies of proton aurora in EMUS disk images
and compare these with contemporaneous but not colocated measurements by MAVEN IUVS and particles and fields instruments. We conclude that multiple mechanisms are required to explain the patchy
proton aurora we observe, including previously suggested radial IMF conditions, as well as newly observed (1) sheath turbulence resulting in charge exchange and ENA deposition in the region of the quasi-parallel interaction, (2) sheath proton deposition in
the vicinity of the solar wind motional electric field
+E pole, and (3) local acceleration of solar wind /
planetary plasma into crustal magnetic field cusps
during particular orientations of the upstream IMF
clock angle. Future study detailing the precise nature
of each mechanism, and particularly the effects of
near radial IMF allowing solar wind proton deposition and ion acceleration across the entire dayside of
the planet, will be essential to understanding the how
these unusual proton aurora formation conditions
affect the present-day state of the planet and determine their consequences for Mars atmospheric evolution.
References:
Amiri+2021:
https://doi.org/10.1007/s11214-021-00868-x
Crismani+2019:
https://doi.org/10.1029/2018JA026251
Deighan+2018:
https://doi.org/10.1038/s41550-018-0538-5
Halekas+2015:
http://dx.doi.org/10.1002/2015GL064693
Holsclaw+2021:
https://doi.org/10.1007/s11214-021-00854-3
Hughes+2019:
https://doi.org/10.1029/2019JA027140